The invention pertains to Raman amplification. More particularly, the invention pertains to broad band fiber optic communication systems utilizing Raman amplification.
In transoceanic fiber optic communication systems, a signal may be transported via optical fiber several thousand kilometers between the transmitter and the receiver. Over such large distances, a light signal transported via fiber optic cable suffers significant degradation. Accordingly, repeater stations comprising amplifiers are placed intermittently in the optical fiber path to amplify the signal in order to compensate for the propagation degradation. Traditionally, in transoceanic transmission system, these repeater stations might be positioned approximately 50 kilometers apart and comprise electronic amplifiers. Accordingly, the light signal needed to be converted from light to electrical, then amplified, and then converted back to light for propagation via the fiber optic cable to the next repeater station or receiver.
More recently, erbium-doped fiber amplification (EDFA) has been used to replace electronic amplifiers. EDFA is a light amplification system that does not require any conversion of the light signals to electrical signals.
Even more recently, Raman amplification systems have been developed for amplifying signals in long-distance fiber optic transmission lines. Raman amplification is a well-known technique for amplifying signals in fiber optic transmission systems. Raman amplification is considered promising for trans-oceanic fiber optic communication systems such as are in use today for international telephonic and other communication applications.
Raman amplification is a well-known phenomenon which will not be described in detail herein. However, in very general terms, one or more pump beams are introduced into a fiber carrying a signal beam. The pump beam(s) should be at a certain wavelength separation from the signal beam. Raman effect is a scattering of the incident photon of the pump beam by a molecule to a lower frequency photon. At the same time, the molecule makes a transition between vibrational states. The incident light acts as a pump to generate the lower frequency light of the signal beam, which is called Stokes wave.
Unlike the aforementioned prior art amplification systems, Raman amplification is a distributed amplification system. That is, the amplification occurs throughout the entire length of the fiber and there is no particular discrete section in the data path to which amplification is limited.
One of the many advantages of Raman amplification is that it can be used in connection with many types of inexpensive, transparent media because it is based on the molecular structure of non crystalline fibers, rather than any particular doping scheme. For instance, Raman amplification can be performed in silica fibers which are relatively low cost. Another advantage of Raman amplification is that Raman amplification can be achieved over a very broad bandwidth, stretching to almost 300 nanometers.
In one common type of scheme, the pump beam(s) are launched to travel in the fiber in the opposite direction to the signal beam(s), i.e., the pump beam(s) are counter-propagating to the signal beam(s). In very long distance fiber optic cable lines, such as the aforementioned transoceanic application, pump beams may be introduced into the fiber at multiple locations between the transmitter and the receiver. However, as previously noted, while the laser sources for the pump beams and the apparatus for coupling the pump beams into the transmission fiber are discrete components, the amplification occurs throughout the entire length of the fiber.
The Raman amplification provided by any given pump beam is not uniform over a range of signal beam wavelengths. FIG. 1 is a graph showing the Raman gain curve for an exemplary pump beam. FIG. 1 shows the gain curve over a frequency span of about 300 nm. By the use of multiple, wavelength-separated, equalized, pump beams, an overall gain curve that is relatively flat over a broad band of signal wavelengths can be provided. For example, FIG. 2A illustrates the gain curves for four closely spaced pump beams centered at about 1425 nm, 1439 nm, 1453 nm and 1467 nm, respectively. These four pump beams such as illustrated in FIG. 2A combine to produce an overall amplification characteristic that is relatively flat over a broad range of wavelengths, such as illustrated in FIG. 2B.
The Raman amplification effect is polarization sensitive. Particularly, when a pump beam and a signal beam are parallelly polarized relative to each other, maximum Raman gain is obtained. On the other hand, when a pump beam and a signal beam are perpendicularly polarized relative to each other, Raman gain is close to zero. Accordingly, it is desirable when using Raman amplification to keep the pump and signal beams parallelly polarized to each other in order to achieve maximum gain. Another possibility is simply to use unpolarized pump and signal beams. This solution decreases gain from the maximum possible achievable gain, but causes all signal beams to experience approximately the same amplification.
Semiconductor lasers are well known in the art. Generally, they are less expensive, smaller, and more reliable than other laser light sources. Accordingly, it is generally desirable to use semiconductor lasers in most applications, including as pump and signal beams for Raman amplification systems. However, semiconductor lasers produce polarized light.
Further, while polarization maintaining optical fibers are known, non-polarization maintaining fibers are much more common and are substantially lower in cost. It is believe that all existing transoceanic fiber optic cables are non-polarization maintaining.
Accordingly, it is an object of the present invention to provide an improved Raman amplification method and apparatus.
It is another object of the present invention to provide a Raman amplification method and apparatus in which polarization dependent gain is minimized.
It is a further object of the present invention to provide a Raman amplification system in which signal beams of disparate wavelengths experience the same amplification.
In accordance with the present invention, a Raman amplification method and apparatus is provided in which a plurality of wavelength division multiplexed signal beams are carried on an optical fiber. A plurality of pump beams are launched into the fiber in a counter propagating direction relative to the signal beams. The pump beams are spaced in wavelength relative to each other such that each pump beam has an adjacent pump beam(s) (i.e., the next lowest frequency and/or next highest frequency pump beam) that is within a wavelength range of it such that the two adjacent beams experience the same polarization evolution as they traverse the fiber. Further, pump beams of adjacent wavelengths are perpendicularly polarized relative to each other.
In this manner, the gain experienced by all signal beams over a broad range of wavelengths is essentially the same.